Certain processor platforms, such as 64-bit ARM® processor platforms, may be implemented with various memory-mapped input/output (MMIO) devices that are not on an enumerable bus such as PCIe (Peripheral Component Interconnect Express). This is comparable to devices on the ISA or LPC buses on x86 platforms. However, the devices on such x86 platforms were well-known with industry-agreed configuration (memory ranges and interrupt lines), whereas on ARM® systems, these “bus-less” devices (termed as such because they are not on an enumerable bus) can be the latest and the most advanced NIC (network interface card) controllers, storage, and so on, whose configuration is described through firmware configuration tables.
For example, in ARM® server platforms, firmware that is compliant with the ACPI (Advanced Configuration and Power Interface) specification (hereinafter referred to as “ACPI firmware”) describes the bus-less devices through DSDT (Differentiated System Description Table), which defines a tree of devices through an interpreted language called AML (ACPI Machine Language). For each bus-less device defined therein, the DSDT contains a descriptor which describes the device's fixed MMIO ranges and interrupt vectors used, as well as its hardware identifier (_HID) and compatible identifier (_CID). These identifiers, hereinafter referred to as device IDs, are used by an operating system (OS) driver to bind to a particular device by matching on a set of IDs supported by the OS driver.
Often, the OS driver is unable to bind to a particular device because the device IDs of bus-less devices have not been standardized and may not be recognizable by the OS driver. For example, a common serial UART (Universal Asynchronous Receiver/Transmitter) known as PL011, may be described using any of the following device IDs, ARMH0011, LNRO000A, LINA000A, or PNP0500 (the format usually conforming to the notation, AAAA####, where AAAA is the assigned vendor, and #### is the assigned device number). In certain situations, the device IDs may be too generic to be recognized. For example, some NICs have device IDs, ETH0000 or SATA0000.
As a result, the OS may not be able to bind a driver to a device if the device ID is too generic, or if a device is known by several device IDs and the one described in the DSDT of the device does not match any of the ones recognized by the OS. In addition, there may be situations where the OS has drivers that can support the device but does not recognize the device ID because the device was developed after the OS was released. In such situations, the OS is unable to bind a driver to the device without an OS patch being installed.